30 out of 568 lettuce samples analysed were norovirus-positive, a rate of 5.3 per cent. Most of these were grown in the UK.

They also found seven out of 310 samples of fresh raspberries had the bug, a rate of 2.3 per cent.

Most of these were imported from other countries, but there was no obvious pattern as to where they had come from.

Traces of the virus were found in supermarket produce (Image: Getty)

10 out of 274 (3.6 per cent) of frozen raspberries were also contaminated.

In most of the positive samples scientists were unable to determine which country the produce had originally come from.

What do the authorities say?

The study was funded by the Food Standards Agency (FSA). It started in 2014, and is still ongoing, with the final result set to be published in 2019.

A FSA spokesperson said: "While the vast majority of the estimated 2.8 million illnesses each year caused by Norovirus are due to close contact between people and touching contaminated surfaces, some of these infections are thought to come from contaminated food.

Some of the contaminated produce was grown in the UK (Image: Getty)

“That’s why we’re funding a large study to assess the contribution made by the food chain to the burden of UK-acquired norovirus infections.

What is norovirus?

Although foodborne infections are estimated as only 2.5 per cent of all norovirus infection on average, research indicates it was responsible for 74,000 cases of food poisoning in the UK in 2009 – estimating it to be the third most common cause of food poisoning in the UK.

Outbreaks of norovirus have been caused by infected oysters and fresh produce such as berries and salad.